The use namespace::autoclean bit is simply good code hygiene, as it removes imported symbols from your class's namespace at the end of your package's compile cycle, including Moose keywords. Once the class has been built, these keywords are not needed. (This is preferred to placing no Moose at the end of your package).

The make_immutable call allows Moose to speed up a lot of things, most notably object construction. The trade-off is that you can no longer change the class definition.

Overriding new is a very bad practice. Instead, you should use a BUILD or BUILDARGS methods to do the same thing. When you override new, Moose can no longer inline a constructor when your class is immutabilized.

There are two good reasons to override new. One, you are writing a MooseX extension that provides its own Moose::Object subclass and a subclass of Moose::Meta::Method::Constructor to inline the constructor. Two, you are subclassing a non-Moose parent.

If you know how to do that, you know when to ignore this best practice ;)

Lazy is good, and often solves initialization ordering problems. It's also good for deferring work that may never have to be done. Make your attributes lazy unless they're required or have trivial defaults.

As described above, you rarely actually need a clearer or a predicate. lazy_build adds both to your public API, which exposes you to use cases that you must now test for. It's much better to avoid adding them until you really need them - use explicit lazy and builder options instead.

The auto_deref feature is a bit troublesome. Directly exposing a complex attribute is ugly. Instead, consider using Moose::Meta::Attribute::Native traits to define an API that only exposes the necessary pieces of functionality.

When using augment and inner, we recommend that you call inner in the most specific subclass of your hierarchy. This makes it possible to subclass further and extend the hierarchy without changing the parents.

It helps ensure that your code will play nice with others, making it more reusable and easier to extend.

Following an accepted set of idioms will make maintenance easier, especially when someone else has to maintain your code. It will also make it easier to get support from other Moose users, since your code will be easier to digest quickly.

Some of these practices are designed to help Moose do the right thing, especially when it comes to immutabilization. This means your code will be faster when immutabilized.

Many of these practices also help get the most out of meta programming. If you used an overridden new to do type coercion by hand, rather than defining a real coercion, there is no introspectable metadata. This sort of thing is particularly problematic for MooseX extensions which rely on introspection to do the right thing.